A quantitative measure of the accuracy of the rate coefficients and the excess energies is a desirable goal of this analysis. There are two major sources of uncertainties: The atomic and molecular data and the solar irradiance. The cross sections and branching ratios used in this analysis come from many different sources; many of them without any error indications. For this reason, we must confine ourselves to a qualitative indication of the reliability of the results. Specifically we give a quality scale in Table II for the data of each mother molecule; A indicating the highest quality of atomic and molecular data and F the lowest quality. The letter B typically means that the threshold is uncertain. For most molecules the cross section at threshold is very small and the rate coefficient for these molecules is therefore not influenced by this uncertainty. For atomic species the cross section is usually large near threshold, but for these species the threshold is known quite accurately. The letter B, therefore, indicates that the rate coefficient is most likely quite accurate, but the excess energy is less accurately known. The letter C usually means that the branching ratios are not well known. This means that the total rate coefficient is very good, but the rate coefficients and the excess energies for the individual branches are less accurate.

Comet nuclei are the most primitive bodies in the solar system. They have been created far away from the early Sun and their material properties have been altered the least since their formation. Thus, the composition and structure of comet nuclei provide the best information about the chemical and thermodynamic conditions in the nebula from which our solar system formed. In this volume, cometary experts review a broad spectrum of ideas and conclusions based on in situ measurement of Comet Halley and remote sensing observations of the recent bright Comets Hale-Bopp and Hyakutake. The chemical character of comet nuclei suggests many close similarities with the composition of interstellar clouds. It also suggests material mixing from the inner solar nebula and challenges the importance of the accretion shock in the outer nebula. The book is intended to serve as a guide for researchers and graduate students working in the field of planetology and solar system exploration. Several special indexes focus the reader's attention to detailed results and discussions. It concludes with recommendations for laboratory investigations and for advanced modeling of comets, the solar nebula, and the collapse of interstellar clouds.

and In the IAU Symposium of 1979 devoted to interstellar molecules [8]. Excellent relevant monographs [ 9. 10] . related timely proceedings [ 11] . and recently published elementary textbooks [12. 13] further help to define the pedagogical scope of molecular astrophysics. A significant financial investment has been made in the establishment of ground- and satellite-based observationai facilities for molecuiar astrophysical studies. In the coming years. a wealth of experimental data is bound to accumulate. in which connection close interactions between observers. astrophysical modeliers. and molecular physicists and chemists can play a helpful role in analysis and interpretation. In view of the increasing pace of activity in the field of molecular astrophysics. and in the apparent absence of relevant international meetings since the Liege 1977 and IAU 1979 Symposia. it was deemed appropriate and timely by the organizers to hold a workshop in 1984. Consequently. the NATO Advanced Research Workshop. "Molecular Astrophysics State of the Art and Future Directions". was organized and held at Bad Wlndshelm. West Germany. from 8 to 14 July 1984. The choice of speakers and subject matter of the Workshop was largely subjective. but designed to include most of the generally accepted areas of molecular astrophysical study. Workers from the fields of radio. infrared. and uv-optlcal observations. astrophysical modelling. laboratory spectroscopy. reaction chemistry. collision physics. and theoretical molecular physics and chemistry. were Invited to present survey lectures In their areas of speciality. In addition.

Comet nuclei are the most primitive bodies in the solar system. They have been created far away from the early Sun and their material properties have been altered the least since their formation. Thus, the composition and structure of comet nuclei provide the best information about the chemical and thermodynamic conditions in the nebula from which our solar system formed. In this volume, cometary experts review a broad spectrum of ideas and conclusions based on in situ measurement of Comet Halley and remote sensing observations of the recent bright Comets Hale-Bopp and Hyakutake. The chemical character of comet nuclei suggests many close similarities with the composition of interstellar clouds. It also suggests material mixing from the inner solar nebula and challenges the importance of the accretion shock in the outer nebula. The book is intended to serve as a guide for researchers and graduate students working in the field of planetology and solar system exploration. Several special indexes focus the reader's attention to detailed results and discussions. It concludes with recommendations for laboratory investigations and for advanced modeling of comets, the solar nebula, and the collapse of interstellar clouds.

A quantitative measure of the accuracy of the rate coefficients and the excess energies is a desirable goal of this analysis. There are two major sources of uncertainties: The atomic and molecular data and the solar irradiance. The cross sections and branching ratios used in this analysis come from many different sources; many of them without any error indications. For this reason, we must confine ourselves to a qualitative indication of the reliability of the results. Specifically we give a quality scale in Table II for the data of each mother molecule; A indicating the highest quality of atomic and molecular data and F the lowest quality. The letter B typically means that the threshold is uncertain. For most molecules the cross section at threshold is very small and the rate coefficient for these molecules is therefore not influenced by this uncertainty. For atomic species the cross section is usually large near threshold, but for these species the threshold is known quite accurately. The letter B, therefore, indicates that the rate coefficient is most likely quite accurate, but the excess energy is less accurately known. The letter C usually means that the branching ratios are not well known. This means that the total rate coefficient is very good, but the rate coefficients and the excess energies for the individual branches are less accurate.